1. Field of the Invention
The present invention relates to a backlight device, and more particularly, to a backlight device that employs a light guide panel having a diffraction pattern.
2. Description of the Related Art
Non-emissive flat panel display devices, such as a liquid crystal display, require a flat fluorescent light source such as a backlight unit. FIG. 1 is a schematic cross-sectional view of a backlight device 10 having a light guide panel 12. Referring to FIG. 1, the backlight device 10 includes the light guide panel 12 having a diffraction pattern 13 that is fine on an upper or lower surface thereof, and a light source 11 on a side of the light guide panel 12.
The light source 11 can be a point light source such as a white light emitting diode (LED) or a line light source such as a cold cathode fluorescent lamp (CCFL). White light emitted from the light source 11 enters the light guide panel 12 through a side of the light guide panel 12 formed of a material such as poly methyl methacrylate (PMMA) having a high optical transmittance, and proceeds in the light guide panel 12 by total reflection. Since the diffraction pattern 13 is formed on an upper surface of the light guide panel 12, a portion of the white light that enters the upper surface of the light guide panel 12 is emitted to the upper surface of the light guide panel 12 due to diffraction caused by the diffraction pattern 13. White light emitted to the upper surface of the light guide panel 12 is uniformly diffused by a diffuser sheet 15, and thus, illuminates a flat panel display device.
The diffraction pattern 13 can be formed by mechanically cutting the light guide panel 12 along its surface, by pressing a stamp against the surface of the light guide panel 12 where the diffraction pattern 13 is to be formed, or using the interference of a laser beam.
However, there is chromatic dispersion when the white light is emitted to the upper surface of the light guide panel 12 through the diffraction pattern 13 since the refractive index and the transmittance of light varies according to the wavelengths of light. FIGS. 2A and 2C are schematic drawings for explaining the chromatic dispersion of light, that is, emission angles of red light R, green light G, and blue light B according to the variation of the period d of the diffraction pattern 13. Here, it is assumed that the refractive index n of the light guide panel 12 is 1.59, the angle of total reflection of light in the light guide panel 12 is 39°, and the center-proceeding angle of light proceeding in the light guide panel 12 is 64.5°. Hence, when the wavelength of the red light is 620 nm, the wavelength of the green light is 530 nm, and the wavelength of the blue light is 460 nm, if the period d of the diffraction pattern 13 is 321 nm, blue light is vertically emitted from the plate light guide panel 12, if the period d of the diffraction pattern 13 is 369 nm, green light is vertically emitted, and if the period d of the diffraction pattern 13 is 453 nm, red light is vertically emitted. Accordingly, when a diffraction pattern having a single period is used, color is separated due to the diffraction pattern since the angles of light emitted from the diffraction pattern are different according to the wavelengths of light.
To address the above problem, as depicted in FIG. 3, a method of using a diffraction pattern 13a having more than two frequencies has been disclosed. For example, a diffraction pattern 13a having a period d of 321 nm and another diffraction pattern 13 having a period d of 369 nm are mixed. Among red light R1, green light G1, and blue light B1 emitted by the diffraction pattern 13 having a period d of 369 nm, the green light G1 is vertically emitted. Also, among red light R2, green light G2, and blue light B2 emitted by the diffraction pattern 13 having a period d of 321 nm, the blue light B2 is vertically emitted. Thus, the chromatic dispersion can be mitigated to some degree due to the mixing of red light, green light, and blue light. However, in this case, a spread angle A of light is increased, and thereby, resulting in reducing front-view brightness. Also, the mitigation effect of chromatic dispersion with respect to emitted light R2, G2, and B1 having a large spread angle A is reduced.